The polar jet stream has a vertical thickness of at least 200 millibars with the core of the jet averaging at about 250 millibars. Either the 200 or 300 mb chart can be used to assess the jet stream. In winter, the 300 mb chart works best and in the summer the 200 mb chart works best.The jet stream is at a higher pressure level (closer to the surface) in the winter because colder air is more dense and is closer to the earth's surface. The average height of each of the most important levels; 1000 mb is near the surface (sea level), 850 mb is near 1,500 meters (5,000 ft), 700 mb is near 3,000 meters (10,000 ft), 500 mb is near 5,500 meters (18,000 ft), 300 mb is near 9,300 meters (30,000 ft).